Polyester film for metal lamination and the use thereof

ABSTRACT

A laminate polyester film to be laminated onto metal plate and molded comprises the first layer made from copoly(ethylene-terephthalate/isophthalate) having a melting point of 205 to 250° C. and a second layer made from a polyester (i) comprising ester units of an aromatic dicarboxylic acid component, an aliphatic dicarboxylic acid component and aliphatic diol components, (ii) containing a titanium compound soluble in the polyester in an amount of 30 to 200 ppm in terms of elemental titanium and (iii) having one or two melting point(s) in the range of 170 to 245° C., and has the maximum peak temperature of loss elastic modulus of higher than 47° C. and 85° C. or lower. This laminate polyester film is suitably used for application such as metal cans for juice and other refreshing drinks that are handled in a cooled state.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a polyester film to be laminated onto metalplate and molded. More specifically, it relates to a polyester film tobe laminated onto metal plate and molded, which exhibits excellentmoldability when a metal plate onto which the film is laminated andsubjected to the deep-drawing or the like in a can-making process andwhich can give metal cans, such as drink cans and food cans, havingexcellent impact resistance, heat resistance, retort resistance andtaste and flavor retention properties.

Metal cans are generally coated on interior and exterior surfacesthereof to be prevented from corrosion. Recently, the development ofmethods for obtaining corrosion resistance without using an organicsolvent has been promoted to simplify production process, improvesanitation and prevent pollution. As one of the methods, coating of ametal can with a thermoplastic resin film has been attempted. That is,studies are under way on a method for making cans by laminating athermoplastic resin film on a plate of a metal such as tin, tin-freesteel or aluminum and then drawing the laminated metal plate. Apolyolefin film or polyamide film has been tried as this thermoplasticresin film but could not satisfy all requirements for moldability, heatresistance, flavor retention property and impact resistance.

On the other hand, a polyester film, particularly a polyethyleneterephthalate film attracts much attention as a film havingwell-balanced properties and there have been made some proposals basedon the polyethylene terephthalate film.

Japanese Laid-open patent application publications 56-10451 and 1-192546disclose a can making material formed by laminating a biaxially orientedpolyethylene terephthalate film onto a metal plate via a low-meltingpolyester adhesive layer.

Although this biaxially oriented polyethylene terephthalate film hasexcellent heat resistance and flavor retention property, its moldabilityis unsatisfactory with the result that the film is whitened (generationof fine cracks) and ruptured when it is subjected to a can makingprocess which inevitably invites large deformation.

Japanese Laid-open patent application publications 1-192545 and 2-57339disclose a can-making material formed by laminating an amorphous orextremely low crystalline aromatic polyester film onto a metal plate.Since this can making material comprises an amorphous or extremely lowcrystalline aromatic polyester film, it is good in moldability butinferior in flavor retention property, and is liable to be embrittledduring the post-treatment such as printing or retort sterilization aftercan-making or during a long-term preservation, and may be changed into afilm that easily cracks by an externally exerted impact.

Japanese Laid-open patent application publication 64-22530 discloses acan making material formed by laminating a heat set biaxially orientedpolyethylene terephthalate film that has been heat set at a low degreeof orientation, onto a metal plate. This film is intended to achieveeffects in a domain intermediate area between the above two differenttypes of films but does not yet attain the low orientation applicable tothe can-making process. Even if it can be processed in a domain with asmall degree of deformation, it is easily embrittled by subsequentprinting and retort treatment for sterilizing the contents of a can andmay be degraded into a film which easily cracks by an impact exertedfrom the outside of the can, like the above amorphous and extremely lowcrystalline film.

U.S. Pat. No. 4,362,775 discloses a polyester film heat-bonded metalplate formed by heat bonding a biaxially oriented polyester film (C) toa metal plate, using an adhesive, the adhesive being a polyblendcomprising 5 to 80% by weight of at least one high melting pointpolyester having a melting point of at least 200° C., and 20 to 95% byweight of at least one low melting point polyester having a meltingpoint of at least 100° C.

Japanese Laid-open patent application publication 6-39981 discloses apolyester film to be laminated onto metal plate and molded, which is alaminate comprising a copolyester layer (A) containing 1% by weight orless of a lubricant having an average particle diameter of 2.5 μm orless and having a melting point of 210 to 245° C. and a glass transitiontemperature of 60° C. or higher, and a polyester layer (B) formed from apolyester composition comprising 99 to 60% by weight of a copolyester(I) composed of ethylene terephthalate having a melting point of 210 to245° C. as a main recurring unit and containing 5 to 30% by weight of afiller having an average particle diameter of 2.5 μm or less and 1 to40% by weight of a polyester (II) composed of butylene terephthalatehaving a melting point of 180 to 223° C. as a main recurring unit.

The polyester composition constituting the polyester layer (B) has twomelting points corresponding to the melting points of two polyesterscontained therein. This publication does not disclose a titaniumcompound soluble in a polyester and loss elastic modulus.

U.S. Pat. No. 5,384,354 discloses a polyester film for lamination onto ametal plate for processing of said metal plate, which polyester film isformed substantially from a composition comprising a molten mixture of

(A) 99 to 60% by weight of a copolyethylene terephthalate having anethylene terephthalate unit as the main recurring unit and having amelting point of 210° C. to 245° C.(component A), and

(B) 1 to 40% by weight of a polybutylene terephthalate or acopolybutylene terephthalate having a butylene terephthalate unit as themain recurring unit and having a melting point of 180 to 223°C.(component B), and a lubricant having an average particle diameter of2.5 μm or less (component C), the film

(i) having a plane orientation coefficient of 0.08 to 0.16,

(ii) having a heat shrinkage of 10% or less at 150° C., and

(iii) having a density of 1,385 g/cm³ or less.

The above molten mixture has two melting points corresponding to themelting points of copolyethylene terephthalate and polybutyleneterephthalate contained therein. The publication does not disclose atitanium compound soluble in a polyester and loss elastic modulus,either.

U.S. Pat. No. 5,618,621 discloses a biaxially oriented laminatedpolyester film for use as a film to be bonded to a metal plate,comprising (A) the first layer formed from the first copolyester whichis composed of ethylene terephthalate unit as a main recurring unit andhas a melting point of 210 to 245° C. and a glass transition temperatureof 50° C. or higher, and (B) the second layer formed from a polyestercomposition containing (B1) a second copolyester which is composed ofethylene terephthalate unit as a main recurring unit and has a meltingpoint of 210 to 245° C., and (B2) the third copolyester which iscomposed of butylene terephthalate unit as a main recurring unit and hasa melting point of 180 to 223° C., the third copolyester being containedin an amount of 1 to 40% by weight based on the total weight of thesecond copolyester and the third copolyester, the second layer being toconstitute a surface to be bonded onto a metal surface when the laminatefilm is laminated on the metal plate. What has been described of themolten mixture in U.S. Pat. No. 5,384,354 can be also applied to thepolyester composition constituting the above second layer.

It is an object of the present invention to provide a polyester film tobe laminated onto metal plate and molded, which has improved properties.

It is another object of the present invention to provide a polyesterfilm to be laminated onto metal plate and molded, which is excellent inmoldability, heat resistance, retort resistance and flavor retentionproperty as well as impact resistance, particularly impact resistance ata room temperature or below.

It is still another object of the present invention to provide apolyester film to be laminated onto metal plate and molded, which issuitably used for such applications that it is handled in a cooledstate, such as metal cans containing juice and refreshing drinks.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, the above objects and advantages ofthe present invention are attained by a laminate polyester film to belaminated onto metal plate and molded, which comprises (A) the firstlayer formed from copoly(ethylene-terephthalate/isophthalate) having amelting point of 205 to 250° C., and (B) the second layer formed from apolyester (i) comprising ester units of an aromatic dicarboxylic acidcomponent, aliphatic dicarboxylic acid component and aliphatic diolcomponents, the aromatic dicarboxylic acid component being terephthalicacid, a combination of terephthalic acid and isophthalic acid, acombination of terephthalic acid and phthalic acid, or a combination ofterephthalic acid, isophthalic acid and phthalic acid, the aliphaticdicarboxylic acid component being at least one member selected from thegroup consisting of adipic acid and sebacic acid, and the aliphatic diolcomponents being ethylene glycol, diethylene glycol and tetramethyleneglycol, (ii) containing a titanium compound soluble in this polyester inan amount of 30 to 200 ppm in terms of elemental titanium and (iii)having one or two melting point(s) in the range of 170 to 245° C.; and

(C) which has the maximum peak temperature of loss elastic modulus ofhigher than 47° C. and 85° C. or lower.

The laminate polyester film of the present invention consists of thefirst layer (A) and the second layer (B) and has a loss elastic modulusspecified in (C), as described above.

The copoly(ethylene-terephthalate/isophthalate) constituting the firstlayer has a melting point of 205 to 250° C. The melting point ispreferably 210 to 245° C., more preferably 215 to 235° C.

The copoly(ethylene-terephthalate/isophthalate) comprises terephthalicacid and isophthalic acid in a total amount of at least 97 mol % of thetotal of all dicarboxylic acid components and ethylene glycol in anamount of at least 97 mol % of the total of all glycol components.

Other dicarboxylic acids other than terephthalic acid and isophthalicacid which may be contained in an amount of 3 mol % or less of the totalof all dicarboxylic acid components are aromatic dicarboxylic acids suchas phthalic acid and naphthalenedicarboxylic acid; aliphaticdicarboxylic acids such as adipic acid, azelaic acid, sebacic acid anddecanedicarboxylic acid; and the like. Other diols other than ethyleneglycol which may be contained in an amount of 3 mol % or less of thetotal of all diol components are aliphatic diols such as diethyleneglycol, triethylene glycol, butane diol, hexane diol and neopentylglycol; alicyclic diols such as cyclohexane dimethanol; and the like.The above other dicarboxylic acids and diols may be used alone or incombination of two or more. Terephthalic acid, isophthalic acid, otherdicarboxylic acid(s) and other diol(s) are copolymerized in such a ratiothat ensures that the melting point of the obtained polymer should be inthe above range. If the melting point of the polymer is lower than 205°C., the heat resistance of the obtained laminate film will deteriorateunfavorably. On the other hand, if the melting point is higher than 250°C., the crystallinity of the obtained polymer will be too high, therebydeteriorating the moldability of the laminate film unfavorably.

The above ratio corresponds to a ratio that ensures the total of theratio of other dicarboxylic acid(s) other than terephthallc acid to thewhole dicarboxylic acid component and the ratio of other diol(s) otherthan ethylene glycol to the whole diol component is about 3 to about 22mol %, although it differs depending on other dicarboxylic acid(s) andother diol(s) used.

The melting point of the copoly(ethylene-terephthalate/isophthalate) isobtained from a melting peak at a temperature elevation rate of 20°C./min using the Du Point Instruments 910 DSC. The amount of a sample isabout 20 mg.

The copoly(ethylene-terephthalate/isophthalate) contains a metal or ametal compound as the polycondensation catalyst. Although the metalcompound is not limited to a particular kind, it is an antimonycompound, germanium compound, tin compound, calcium compound, magnesiumcompound or the like.

The intrinsic viscosity of thecopoly(ethylene-terephthalate/isophthalate) is preferably 0.52 to 0.80dl/g, more preferably 0.54 to 0.70 dl/g, particularly preferably 0.57 to0.65 dl/g. The intrinsic viscosity is obtained from the viscosity of asolution measured in o-chlorophenol at 35° C.

The polyester constituting the second layer is composed of ester unitsof an aromatic dicarboxylic acid component, aliphatic dicarboxylic acidcomponent and aliphatic diol components. That is, the polyester iscomposed of ester units of an aromatic dicarboxylic acid and aliphaticdiol and ester units of an aliphatic dicarboxylic acid and aliphaticdiol.

The aromatic dicarboxylic acid is either one of terephthalic acid alone,a combination of terephthalic acid and isophthalic acid, a combinationof terephthalic acid and phthalic acid and a combination of terephthalicacid, isophthalic acid and phthalic acid. The aliphatic dicarboxylicacid is either one or both of adipic acid and sebacic acid. Thealiphatic diol is a combination of ethylene glycol, diethylene glycoland tetramethylene glycol.

The polyester constituting the second layer (B) further contains atitanium compound soluble in this polyester in an amount of 30 to 200ppm in terms of elemental titanium. Although the titanium compound isgenerally derived from a polyester polymerization catalyst, the aboveamount is relatively larger than a required amount of a polymerizationcatalyst.

The amount of the elemental titanium is preferably 40 to 180 ppm, morepreferably 50 to 160 ppm. The polyester constituting the second layer(B) may further contain an antimony compound, germanium compound, tincompound, calcium compound, magnesium compound or the like in additionto the titanium compound soluble in the above polyester.

Further, the polyester constituting the second layer (B) has one or twomelting point(s) in the range of 170 to 245° C., preferably 190 to 240°C. The measurement method of the melting point is the same as that ofthe copoly(ethylene-terephthalate/isophthalate) constituting the firstlayer (A).

The aromatic dicarboxylic acid component of the polyester constitutingthe second layer (B) is preferably a combination of terephthalic acidand isophthalic acid, a combination of terephthalic acid and phthalicacid, or a combination of terephthalic acid, isophthalic acid andphthalic acid. The above acid(s) other than terephthalic acid in theabove combinations, that is, isophthalic acid in the first combination,phthalic acid in the second combination, and isophthalic acid andphthalic acid in the third combination is(are) contained in a proportionof 20 mol % or less of the total of the aromatic dicarboxylic acidcomponent. This proportion is more preferably about 1 to 12 mol %.

Moreover, the aliphatic dicarboxylic acid component of the polyesterconstituting the second layer (B) is preferably adipic acid.

As for the aromatic dicarboxylic acid component and the aliphaticdicarboxylic acid component constituting the polyester of the secondlayer (B), the aliphatic dicarboxylic acid component is preferablycontained in a proportion of 12 molt or less of the total of thealiphatic dicarboxylic acid component and the aromatic dicarboxylic acidcomponent. The proportion is more preferably 1 to 8 mol %.

As for the proportion of the aliphatic diol component, the proportionsof ethylene glycol, diethylene glycol and tetramethylene glycol are 63to 98.5 molt, 1 to 2 mol % and 0.5 to 36 mol % of the total of allaliphatic diol components, respectively. The proportions are morepreferably 63 to 79.5 mol %, 1 to 2 molt and 19.5 to 36 molt,respectively. The polyester forming the second layer (B) can be producedby polycondensing the aromatic dicarboxylic acid component, aliphaticdicarboxylic acid component and aliphatic diol component which have theabove compositions in accordance with a direct esterification or esterexchange method which is known per se. Alternatively, two or morepolyesters prepared in advance are melt mixed in such a ratio thatensures the above composition to produce the polyester. In eithermethod, as the diethylene glycol component is produced in a reactionsystem during a is polycondensation reaction in the production of apolyester containing ethylene glycol as a diol component, it is notalways necessary to prepare the diethylene glycol component as amaterial for producing the polyester.

In accordance with the former method out of the above methods, apolyester containing a titanium compound soluble in the polyester in anamount of 30 to 200 ppm in terms of elemental titanium and having onemelting point of 170 to 245° C. can be obtained.

The latter method out of the above methods, that is, a method in whichtwo or more polyesters are prepared in advance and melt mixed in such aratio that ensures the above composition is more preferable as a methodfor producing the polyester for the second layer (B) in the presentinvention. According to this method, a polyester containing a titaniumcompound soluble in the polyester in an amount of 30 to 200 ppm in termsof elemental titanium and having one or two melting point(s) of 170 to245° C. can be obtained.

In this melt mixing method, the polyester forming the second layer canbe advantageously produced as a molten mixture comprisingcopoly(ethylene-terephthalate/isophthalate) having a melting point of210 to 245° C. and copoly(ethylene terephthalate/adipate) having amelting point of 170 to 223° C. As a matter of course, a desired moltenmixture can be produced from a combination of polyesters other than theabove combination.

If the melting point of the copoly(tetramethylene terephthalate/adipate)is lower than 210° C., the heat resistance of the obtained laminate filmwill deteriorate unfavorably. On the other hand, if the melting point ishigher than 245° C., the crystallinity of the obtained polymer willbecome too high, thereby impairing the moldability of the filmunfavorably.

The measurement method of the melting point is the same as that of thecopoly(ethylene-terephthalate/isophthalate) constituting the first layer(A).

The copoly(ethylene-terephthalate/isophthalate) of the second layer (B)can be produced using a soluble titanium compound catalyst or othercatalyst such as an antimony compound, germanium compound or the like.

The intrinsic viscosity of thecopoly(ethylene-terephthalate/isophthalate) is preferably 0.52 to 0.80dl/g, more preferably 0.54 to 0.70 dl/g, particularly preferably 0.57 to0.65 dl/g.

The copoly(tetramethylene terephthalate/adipate) has a melting point of170 to 223° C.

If the melting point is lower than 170° C., the heat resistance of theobtained laminate film will deteriorate unfavorably. The melting pointof a polybutylene terephthalate homopolymer is about 223° C.

This melting point is preferably 180 to 215° C., more preferably 180 to205° C.

The measurement method of this melting point is also the same asdescribed above.

The copoly(tetramethylene terephthalate/adipate) particularly preferablycontains an adipic acid component in an amount of 5 to 40 mol % of thetotal of the terephthalic acid component and the adipic acid component.

The copoly(tetramethylene terephthalate/adipate) is preferably producedusing a soluble titanium compound as a catalyst. Illustrative examplesof the soluble titanium compound used in the present invention includeorganic titanates such as tetra-n-butyl titanate, tetra-n-propyltitanate, tetraisopropyl titanate, tetraethyl titanate and tetramethyltitanate and hydrolyzed products thereof; hydrolyzed products oftitanium tetrachloride and titanium sulfate; inorganic titaniumcompounds such as zinc titanium fluoride, potassium titanium fluorideand cobalt titanium fluoride; titanium compounds usually used as acatalyst in the production of a polyester, such as titanium acetate,titanium oxalate and potassium titanate oxalate, and the like. Of these,tetra-n-butyl titanate is preferred. These titanium compounds may beused in combination of two or more. The copoly(tetramethyleneterephthalate/adipate) may further contain an antimony compound,germanium compound, tin compound, calcium compound, magnesium compoundor the like in addition to the soluble titanium compound. Thecopoly(tetramethylene terephthalate/adipate) preferably has terminalcarboxyl groups in an amount of 1 to 40 eq/10⁶ g. The terminal carboxylgroups are measured in accordance with a Conix method (Macromol, Chem.vol.26, pp.226 (1958)).

The copoly(tetramethylene terephthalate/adipate) preferably containsonly 0.5 wt % at most of oligomers. The content of the oligomers is morepreferably 0.4 wt % at most. If the content of the oligomers is largerthan 0.5 wt %, an increase in the total amount of the oligomers afterthe formation of a film will be large, the total amount of the oligomersextracted after a retort treatment will increase, and flavor retentionproperty will deteriorate unfavorably.

The intrinsic viscosity of the copoly(tetramethyleneterephthalate/adipate) is preferably 0.70 to 2.0 dl/g, more preferably0.80 to 1.70 dl/g, particularly preferably 0.85 to 1.5 dl/g.

The above molten mixture which is the polyester forming the second layer(B) comprises 99 to 60 wt % of thecopoly(ethylene-terephthalate/isophthalate) and 1 to 40 wt % of thecopoly(tetramethylene terephthalate/adipate).

When the proportion of the copoly(ethylene-terephthalate/isophthalate)is larger than 99 wt % and the proportion of the copoly(tetramethyleneterephthalate/adipate) is smaller than 1 wt %, it is difficult to obtaina laminate film whose impact resistance at low temperatures is improvedto a desired extent, while when the proportion of thecopoly(ethylene-terephthalate/isophthalate) is smaller than 60 wt % andthe proportion of the copoly(tetramethylene terephthalate/adipate) islarger than 40 wt %, it is difficult to obtain a laminate film havingsatisfactory heat resistance and impact resistance.

The above molten mixture of thecopoly(ethylene-terephthalate/isophthalate) and thecopoly(tetramethylene terephthalate/adipate) preferably has a reactionrate (EE, %) defined by the following equation of 3% or higher.

    EE (%)=SB/(SA+SB+SC)×100

wherein SA, SB and SC are integral values of peak A, peak B and peak Cclearly distinguishable from one another in an ¹ H-NMR chart,respectively, the peak A is based on proton HA having a structurerepresented by the following formula: ##STR1## the peak B is based onproton H_(B) having a structure represented by the following formula:##STR2## and the peak C is based on proton H_(C) having a structurerepresented by the following formula. ##STR3##

The molten mixture of the copoly(ethylene-terephthalate/isophthalate)and the copoly(tetramethylene terephthaiate/adipate) which is apolyester forming the second laser (B) has one melting point differentfrom the melting points of these copolyesters or two melting points of amelting point different from the melting points of these copolyestersand the melting point of either one of the copolyesters. When the moltenmixture has one melting point, a reaction (ester-ester exchangereaction) between the copolymers proceeds at a relatively high rate,while when the molten mixture has two melting points, the reaction doesnot proceed so far. In this case, the melting point of either one of thecopolyesters is substantially equal to the melting point of thecopoly(ethylene-terephthalate/isophthalate) which is used in largequantities in many cases.

The ester-ester exchange reaction proceeds advantageously when thepolyester of the second layer (B) contains a relatively large amount ofthe soluble titanium compound, as described above. When both of theabove copolyesters do not contain a sufficient amount of the titaniumcompound, it is preferred to add the soluble titanium compound at thetime of melt mixing of the copolyesters.

That is, in the case of a polyester containing the soluble titaniumcompound in an amount of less than 30 ppm in terms of elementaltitanium, when the film is laminated onto a metal plate and deep-drawn,particularly at an extremely high draw ratio, or subjected tohigher-order postprocessing, sufficient moldability and impactresistance cannot be obtained, whereby cracks are produced or a filmwhitening phenomenon occurs at the time of molding unfavorably. In thecase of a polyester containing the soluble titanium compound in anamount of more than 200 ppm, on the other hand, the melting heatstability of the polymer degrades, and a reduction in molecular weightat the time of molding is large. When the film is laminated onto a metalplate and deep-drawn, moldability and impact resistance deteriorateunfavorably.

Surprisingly, it has been found that the ester-ester exchange reactionproceeds advantageously when at least one of the copolyesters containsan antioxidant. The antioxidant is preferably the one which issubstantially harmless to the human body and stable when a polyestercontaining an antioxidant is coated under heating onto a metal plate,namely, at 240° C.

Illustrative examples of the antioxidant include phenol group-containingcompounds such astetrakis-(methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate)methane,stearyl-p-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,2,2'-methylene-bis(4-methyl-6-tert-butylphenol),2,2'-methylene-bis(4-ethyl-6-tert-butylphenol),4,4'-thiobis(3-methyl-6-tert-butyl-phenol),4,4'-butylidene-bis(3-methyl-6-tert-butylphenol),3,9-bis(1,1-dimethyl-2-(β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy)ethyl)-2,4,8,10-tetraoxaspyro(5,5)undecane,1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,1,3,5-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)-S-triazine-2,4,6-(1H,3H,5H)trionand tocopherol; sulfur-containing compounds such asdilauryl-3,3'-thiodipropionate, dimyristyl-3,3'-thiodipropionate anddistearyl-3,3'-thiodipropionate; and phosphorus-containing compoundssuch as cyclic neopentanetetraylbis(octadecylphosphite),tris(nonylphenyl)phosphite and cyclicneopentanetetraylbis(2,4-di-tert-butylphenyl)phosphite.

These antioxidants may be used alone or in combination of two or more.At least one phenol group-containing compound is preferably contained.Sulfur-containing compounds and phosphorus-containing compounds are usedas secondary antioxidants. When a sulfur-containing compound or aphosphorus-containing compound is used in conjunction with a phenolgroup-containing compound which serves as a primary antioxidant, asynergistic effect can be obtained.

Of these,tetrakis-(methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate)methanewhich is a phenol group-containing compound is preferred.

The antioxidant is preferably contained in the polyester of the secondlayer (B) in an amount of 5 wt % or less, more preferably 0.01 to 5 wt%. When the proportion is less than 0.01 wt %, the effect of theantioxidant added is hardly observed, while when the proportion is morethan 5 wt %, further effect of the antioxidant is hardly obtained.

The antioxidant may be added at the same time when raw materials arecharged or just before the raw materials are supplied to apolymerization tank in an esterification reaction or ester exchangereaction for the synthesis of the copolyester. It may be directly addedto the polymerization tank for the copolyester after polymerization ormixed with the copolyester by an extruder, heating roll, Banbury mixeror kneader.

Preferably, the laminate polyester film of the present invention furthercontains a lubricant in an amount of 1 wt % or less to improve windingproperty (handling property and continuous length) in the filmproduction process.

This lubricant is preferably contained in at least either one of thecopoly(ethylene-terephthalate/isophthalate) constituting the first layer(A) and the polyester constituting the second layer (B).

The lubricant may be inorganic or organic while it is preferablyinorganic. Inorganic lubricants include silica, alumina, titaniumdioxide, calcium carbonate, barium sulfate and the like. Organiclubricants include cross-linked polystyrene, cross-linked silicone resinparticles and the like. The lubricant, inorganic or organic, preferablyhas an average particle diameter of 2.5 μm or less, more preferably 0.05to 2.2 μm. When the average particle diameter of the lubricant is morethan 2.5 μm, a pin hole is produced by a coarse lubricant particle (suchas a particle having an average particle diameter of 10 μm or more) as astarting point or a rupture occurs in some case, in a portion deformedby a can-making process such as deep drawing or the like, unfavorably. Alubricant which is preferred from a viewpoint of resistance to pin holeis a monodisperse spherical lubricant having an average particlediameter of 2.5 μm or less and a particle diameter ratio (majordiameter/minor diameter) of 1.0 to 1.2. Illustrative examples of thelubricant include spherical silica, spherical zirconium oxide, sphericalsilicone resin particles and the like.

Although the content of the lubricant is preferably 0.005 to 5 wt %, itmay be determined by winding property in the film production process.Generally speaking, it is preferred that a lubricant having a largeparticle diameter should be contained in small quantities and alubricant having a small particle diameter should be contained in largequantities. For example, monodisperse silica having an average particlediameter of 2.0 μm is preferably contained in an amount of ca. 0.05 wt %and titanium oxide having an average particle diameter of 0.3 μm ispreferably contained in an amount of ca. 0.4 wt %. An opaque film can beobtained by deliberately increasing the content of the fine particles.For example, by adding titanium dioxide in an amount of 5 to 40 wt %,preferably 10 to 20 wt %, a white film can be obtained.

The lubricant is not limited to the above externally added particles.Internally deposited particles obtained by depositing part or all of thecatalyst used in the production of the polyester in a reaction process.The externally added particles and the internally deposited particlescan be used in conjunction.

The laminate polyester film of the present invention consisting of thefirst layer (A) and the second layer (B) has the maximum peaktemperature of loss elastic modulus at a temperature ranging from higherthan 47° C. to 85° C. or lower.

If the maximum peak temperature of loss elastic modulus is higher than85° C., sufficient low-temperature impact resistance cannot be obtained,while if it is lower than 47° C., heat resistance may deteriorate andconversely impact resistance may be lost.

The maximum peak temperature of loss elastic modulus is preferably 50 to85° C., more preferably 50 to 75° C., particularly preferably 50 to 70°C.

The maximum peak temperature of loss elastic modulus depends on thecomposition of the polyester forming the second layer (B), the blendratio of the copolyesters when the polyester is a molten mixture of thecopolyesters, and stretch ratio and stretch temperature when the film isbiaxially oriented.

The laminate polyester film of the present invention can be produced,for example, by melting a polyester forming the first layer (A) and apolyester forming the second layer (B) separately, co-extruding thepolyesters into films, laminating them to fuse together beforesolidification, biaxially orienting the resulting laminate and heatsetting it, or by melting the polyesters separately, extruding them intofilms, laminating them together and then stretching or stretching andthen laminating them together, and heat setting the resulting laminatebefore or after lamination after stretching.

The laminate polyester film to be laminated onto metal plate and moldedof the present invention may be an unstretched film but preferably abiaxially oriented film which is stretched biaxially and heat set. Inthis case, the refractive index in the thickness direction of the firstlayer (A) is preferably 1.490 to 1.550, more preferably more than 1.505and 1.540 or less. If the refractive index is too low, moldability willbe unsatisfactory, while if it is too high, the film has an amorphousstructure with the consequence that heat resistance may deteriorate.

The laminate polyester film to be laminated onto metal plate and moldedof the present invention preferably has a thickness of 6 to 75 μm, morepreferably 10 to 75 μm, particularly preferably 15 to 50 μm. If thethickness is smaller than 6 μm, a rupture or the like is liable to occurupon molding. If the thickness is larger than 75 μm, the quality will beexcessively high uneconomically.

The ratio (T_(A) /T_(B)) of the thickness T_(A) of the first layer (A)to the thickness T_(B) of the second layer (B) is preferably 0.02 to1.5, more preferably 0.02 to 0.67, the most preferably 0.04 to 0.67,particularly preferably 0.04 to 0.25. Stated more specifically, in thecase of a polyester film having a thickness of 20 μm, the first layer(A) should be made as thick as 0.5 to 15 μm, preferably 1 to 10 μm, morepreferably 1 to 4 μm.

The extraction of the laminate polyester film to be laminated onto metalplate and molded of the present invention is preferably 0.5 mg/inch² orless when it is extracted with ion exchange water at 121° C. for 2hours.

As the metal plate to be laminated with the laminate polyester film tobe laminated onto metal plate and molded of the present invention,particularly a metal plate for making cans, a tinplate, tin-free steelor aluminum plate is suitable.

The lamination of the film on the metal plate can be carried out inaccordance with the following methods (a) or (b).

(a) After the film is laminated on the metal plate heated at atemperature higher than the melting point of the film, the metal plateis cooled to make the surface layer portion (thin layer portion) of thefilm in contact with the metal plate amorphous to allow the film to bebonded thereto.

(b) The film is primer coated with an adhesive on one surface andlaminated on the metal plate in such a manner that the surface comesinto contact with the metal plate. Known resin adhesives such as epoxyadhesives, epoxy-ester adhesives and alkyd adhesives can be used as theadhesive.

When the laminate polyester film of the present invention is to belaminated on the metal plate, it is laminated in such a manner that thesecond layer (B) comes into contact with the metal plate.

Further, in the polyester film of the present invention, an additionallayer may be laminated between the first layer (A) and the second layer(B) or on either one of the layers as required.

The metal plate laminated with the laminate polyester film of thepresent invention is suitably used to make a metal can by deep-drawing.

The following examples are given to further illustrate the presentinvention.

In the present invention, the physical property values of the film weremeasured and evaluated in the following manners.

In the following measurement and evaluation methods, in the measurementconditions or evaluation criteria of Examples 4 to 16 and ComparativeExamples 6 to 12, measurement conditions or criteria different fromthose of Examples 1 to 3 and Comparative Examples 1 to 5 are shownwithin parentheses.

(1) Content of titanium compound soluble in polyester

A sample polymer is dissolved in a solvent (such as a mixture solutionof chloroform and hexafluoroisopropanol) and an insoluble titaniumcompound (such as titanium oxide particles) is centrifuged, a residualsupernatant is evaporated to obtain a dry solid polymer. A solutionprepared by wet dissolving this polymer with a sulfuric acid/nitric acidmixture solution (1/1) is subjected to ICP emission spectral analysis todetermine the quantity of titanium metal.

(2) Maximum peak temperature of loss elastic modulus

Using a dynamic visco-elasticity measuring instrument, the maximum peaktemperature of loss elastic modulus is measured at a frequency of 10 Hzand a dynamic displacement of ±25×10⁻⁴ cm.

(3) Content of oligomers

10 Mg of a sample is dissolved in 2 ml of a mixture solution ofchloroform and hexafluoroisopropanol (volume ratio of 3/2), andchloroform is further added to dilute the solution to an amount of 10ml. The total quantity of oligomers contained in the solution isdetermined by gel permeation chromatography (column: TSKgel.G2000H8 7.5mmID×60 cm of Tosoh Corporation) using chloroform as a solvent and thetotal quantity of oligomers of monomer to pentomer is taken as thecontent of oligomers.

(4) Amount of extract with ion exchange water

3.10 Ml of water is used per 1 inch² of the surface area of one side ofa polyester film to carry out an extraction test at 121° C. for 2 hours.The obtained extract is evaporated and dried to obtain a residue. Thisresidue is weighed to obtain the quantity of an extract per 1 inch² ofthe surface area of one side of the film.

(5) Lamination property

The film is laminated on a tin-free steel plate heated at a temperaturehigher than the melting point of a polyester and then cooled to obtain acoated steel plate. The lamination property of this coated steel plateis evaluated based on the following criteria.

(A) criteria based on air bubbles and wrinkles

∘: No air bubbles and wrinkles is observed.

Δ: A few air bubbles and wrinkles are observed per 10 m of length. ×:Many air bubbles and wrinkles are observed.

(B) criteria based on heat shrinkage ∘: shrinkage of less than 2 %

Δ: shrinkage of 2% or more and less than 5%

×: shrinkage of 5% or more

(6) Deep drawability-1

A tin-free steel plate laminated with a film is molded into a containerhaving a diameter of 50 mm (55 mm), a height of 150 mm and a seamlessside (to be referred to as "can" hereinafter), by a dice and punch. Thiscan is observed and tested as follows and evaluated based on thefollowing criteria.

∘: There is no abnormality in the film, and whitening or rupture of themolded film is not observed.

Δ: Whitening of the film is observed at a top portion of the can.

×: Rupture is observed in part of the film.

(7) Deep drawability-2

⊚: The film is molded without abnormality and exhibits 0.005 mA or lessin an anti-corrosion test on the inner film surface of the can (when thecan is charged with a 1% NaCl aqueous solution, an electrode is insertedinto the can and a voltage of 6V is applied with the can body used as ananode, and a current value is measured. This test is called "ERV test"hereinafter).

∘: The film exhibits 0.005 to 0.05 mA in the ERV test.

Δ: The film exhibits 0.05 to 0.1 mA In the ERV test.

×: The film exhibits 0.1 mA or more in the ERV test and a crack isobserved when electricity passes is magnified for observation.

(8) Adhesion

Well deep-drawn cans are filled with water and subjected to a retorttreatment at 120° C. for 90 minutes in a steam sterilizer. Thereafter,the cans are kept at 50° C. for 3 months. The resulting cans are cutcross to observe the adhesion state of the film.

∘: The film is firmly adhered to the metal surface and does not peel offeven when it is cut cross.

Δ: Slight reduction in the adhesion of the film by cutting cross isobserved.

×: The film peels off when it is cut cross.

(9) Corrosion resistance

Well deep-drawn cans are filled with a 5% acetic acid aqueous solutionand held at 501C for 2 weeks. 10 Cans for each test are visuallyobserved and evaluated for the formation of rust in the metal plates.

∘: The formation of rust is not observed in all 10 cans.

Δ: The formation of rust is observed in 1 to 5 cans.

×: The formation of rust is observed in 6 or more cans.

(10) Impact resistance

Well deep-drawn cans are filled with water and cooled at 10° C. 10 Cansfor each test are dropped on the polyvinyl chloride (abbreviated as"PVC" hereinafter) resin tiled floor from a height of 2 m (30 cm), andthe content of the can after one month preservation is evaluatedaccording to an ERV test.

∘: All 10 cans exhibit 0.1 mA (0.2 mA) or less.

Δ: 1 to 5 cans exhibit 0.1 mA (0.2 mA) or more.

×: 6 or more cans exhibit 0.1 mA (0.2 mA) or more, or a crack isobserved in the film after dropping.

(11) Resistance to thermal embrittlement

Well deep-drawn cans are held at 210° C. for 10 minutes (200° C.×5minutes) and then the impact resistance thereof is evaluated asdescribed in (9).

∘: All 10 cans exhibit 0.1 mA or less.

Δ: 1 to 5 cans exhibit 0.1 mA or more.

×: 6 or more cans exhibit 0.1 mA or more, or a crack is observed in thefilm after it is heated at 210° C. for 5 minutes.

(12) Retort resistance

Well deep-drawn cans are filled with water, subjected to a retorttreatment at 120° C. for 90 minutes (120° C. for 1 hour) in a steamsterilizer and thereafter, kept at 50° C. for 30 days. 10 Cans for eachtest are dropped on the polyvinyl chloride tiled floor from a height of50 cm (1 m) and an ERV test is carried out on the cans.

∘: All 10 cans exhibit 0.1 mA (2 mA) or less.

Δ: 1 to 5 cans exhibit 0.1 mA (2 mA) or more.

×: 6 or more cans exhibit 0.1 mA (2 mA) or more, or a crack is observedin the film after dropping.

(13) Taste retention property-1

A cut sheet of 21 cm×30 cm is obtained by cutting a polyester film andthis sample film is immersed in 300 ml of ion exchange water and kept atnormal temperature (20° C.) for 3 months. Thirty panelists taste thisimmersion solution to compare it with ion exchange water as referenceand evaluate it based on the following criteria.

⊚: 4 or less of 30 panelists judge that the immersion solution tastesworse than the reference solution.

∘: 5 to 6 of 30 panelists judge that the immersion solution tastes worsethan the reference solution.

Δ: 7 to 9 of 30 panelists Judge that the immersion solution tastes worsethan the reference solution.

×: 10 or more of 30 panelists judge that the immersion solution tastesworse than the reference solution.

(14) Flavor retention property-1

Well deep-drawn cans are filled with cider and sealed. They are kept at37° C for 30 days and opened. A change in flavor is checked by a sensorytest and evaluated based on the following criteria.

∘: There is no change in flavor.

Δ: A slight change in flavor is perceived.

×: A change in flavor is perceived.

(15) Taste retention property-2

A change in taste is checked by an organoleptic test in the same manneras in (14) above and evaluated based on the following criteria.

∘: There is no change in taste.

Δ: A slight change in taste is perceived.

×: A change in taste is perceived. ps (16) Ester exchange rate

About 10 mg of the polyester composition is cut off from a sample filmand dissolved in a mixture solvent of CDCl₃ and CF₃ COOD, and the esterexchange rate is calculated from the integral values of peaks SA, SB andSC measured by 600 Mhz¹ H-NMR based on the following equation.

    EE (%)=SB/(SA+SB+SC)×100

SA, SB and SC are integral values of peak A, peak B and peak C clearlydistinguishable from one another in an ¹ H-NMR chart, respectively, thepeak A is based on proton H_(A) having a structure represented by thefollowing formula: ##STR4## the peak B is based on proton H_(B) having astructure represented by the following formula: ##STR5## and the peak Cis based on proton H_(C) having a structure represented by the followingformula. ##STR6##

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 TO 4

Copolybutylene terephthalate (copolyester (I)) prepolymers having anintrinsic viscosity of 0.70 and prepared by copolymerizing copolymercomponents shown in Table 1 was produced by melt polycondensation usingtetrabutoxy titanate as a titanium compound, which was used as acatalyst, in such an amount that the amount of titanium metal became avalue shown in Table 1. Thereafter, each of the copolyester (I)prepolymers was pelletized, dried at 160° C. for 4 hours,precrystallized, and then polymerized in a solid state in a nitrogen gasstream using a normal pressure fluidized bed solid-phase polymerizationapparatus to obtain a copolyester (I) having an intrinsic viscosity of1.1 and an oligomer content of 0.40 wt % (0.50 wt % for ComparativeExample 3).

Polyester compositions for a polyester layer (B) (adhesive layer) wereprepared by melt blending the thus obtained copolyester (I) and anaromatic polyester (II) prepared separately (having a composition andmelting point shown in Table 1 and containing 0.3 wt % of sphericalsilica having an intrinsic viscosity of 0.70 and an average particlediameter of 1.0 μm). Separately, aromatic polyesters (III) for apolyester layer (A) (intrinsic viscosity of 0.70) having a composition,a melting point and a glass transition temperature shown in Table 1 wereprepared. They were then dried and molten by a commonly used methodindependently and thereafter, co-extruded from adjacent dies to belaminated, fused together, and solidified by quenching to prepareunstretched laminate films. Then, each of the unstretched laminate filmswas stretched to 3.0 times at 100° C. in a longitudinal direction and,then, to 3.0 times in a transverse direction, while changing thetemperature and the amount of hot air to be blown thereto, and heat setat 180° C. to obtain biaxially oriented laminate polyester films.

COMPARATIVE EXAMPLE 5

A biaxially oriented film was obtained in the same manner as in Example1 except that only the aromatic polyester (III) of Example 1 was meltextruded into a 25 μm-thick single-layer film.

                                      TABLE 1                                     __________________________________________________________________________    Polyester layer (A)                                                           Aromatic polyester (III)                                                      Basic         Copolymer component                                                                         Tm Tg                                                 component Composition                                                                         Mol %   ° C.                                                                      ° C.                                    __________________________________________________________________________    Ex. 1                                                                              PET      IA    12      229                                                                              73                                             Ex. 2                                                                             PET       IA    12      229                                                                              73                                             Ex. 3                                                                             PET       IA    12      229                                                                              73                                             C. Ex. 1                                                                          PET       IA    12      229                                                                              73                                             C. Ex. 2                                                                          PET       IA    12      229                                                                              73                                             C. Ex. 3                                                                          PET       IA    12      229                                                                              73                                             C. Ex. 4                                                                          PET       IA    12      229                                                                              73                                             C. Ex. 5                                                                          PET       IA    12      229                                                                              73                                             __________________________________________________________________________    Polyester layer (B)                                                           Copolyester (I)                                                                         Copolymer Amount of                                                 Basic     Component titanium                                                                           Concentration of                                                                      Weight                                           component                                                                           Composition                                                                         Mol %                                                                             metal ppm                                                                          Antioxidant wt %                                                                      wt %                                         __________________________________________________________________________    Ex. 1                                                                             PBT   AA    25  120  0.2     40                                           Ex. 2                                                                             PBT   AA    25   50  0.2     40                                           Ex. 3                                                                             PBT   SA    20  120  0.2     40                                           C. Ex. 1                                                                          PBT   AA    25   8   0.2     40                                           C. Ex. 2                                                                          PBT   AA    25  220  0.2     40                                           C. Ex. 3                                                                          PBT   IA    25  120  0.2     40                                           C. Ex. 4                                                                          PBT   None   0  120  0.2     40                                           C. Ex. 5                                                                          --    --    --  --   --      --                                           __________________________________________________________________________    Polyester layer (B)                                                           Aromatic Polyester (II)                                                       Basic         Copolymer component                                                                         Tm Weight                                             component Composition                                                                         Mol %   ° C.                                                                      wt %                                           __________________________________________________________________________    Ex. 1                                                                             PET       IA    12      229                                                                              60                                             Ex. 2                                                                             PET       IA    12      229                                                                              60                                             Ex. 3                                                                             PET       IA    12      229                                                                              60                                             C Ex. 1                                                                           PET       IA    12      229                                                                              60                                             C Ex. 2                                                                           PET       IA    12      229                                                                              60                                             C Ex. 3                                                                           PET       IA    12      229                                                                              60                                             C Ex. 4                                                                           PET       IA    12      229                                                                              60                                             C Ex. 5                                                                           --        --    --      -- --                                             __________________________________________________________________________     Ex.: Example                                                                  C. Ex.: Comparative Example                                              

In Table 1, IA stands for isophthalic acid, AA for adipic acid, SA forsebacic acid, PET for polyethylene terephthalate, and PBT forpolybutylene terephthalate. Tg indicates a glass transition temperatureand Tm a melting point.

The thus obtained biaxially oriented film was laminated on a tin-freesteel plate heated at a temperature higher than the melting point of thepolyester and cooled to prepare a coated steel plate. This coated steelplate was further molded into a can having a seamless side by a dice andpunch.

The lamination property, moldability, adhesion and other characteristicproperties of the can are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Lamina-                        Resistance                                     tion    Deep                                                                              Deep               to thermal                                                                              Taste                                property                                                                              draw-                                                                             draw-    Corrosion                                                                          Impact                                                                             embrittle-                                                                         Retort                                                                             retention                            (A)   (B)                                                                             ability-1                                                                         ability-2                                                                         Adhesion                                                                           resistance                                                                         resistance                                                                         ment resistance                                                                         property-1                           __________________________________________________________________________    Ex. 1                                                                             ◯                                                                   ◯                                                                   ◯                                                                     ◯                                                                     ◯                                                                      ◯                                                                      ⊚                                                                   ◯                                                                      ◯                                                                      ⊚                     Ex. 2                                                                             ◯                                                                   ◯                                                                   ◯                                                                     ◯                                                                     ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ⊚                     Ex. 3                                                                             ◯                                                                   ◯                                                                   ◯                                                                     ◯                                                                     ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ⊚                     C. Ex. 1                                                                          ◯                                                                   ◯                                                                   ◯                                                                     Δ                                                                           Δ                                                                            Δ                                                                            X    Δ                                                                            X    ◯                        C. Ex. 2                                                                          ◯                                                                   ◯                                                                   ◯                                                                     Δ                                                                           ◯                                                                      ◯                                                                      Δ                                                                            X    Δ                                                                            Δ                              C. Ex. 3                                                                          ◯                                                                   ◯                                                                   ◯                                                                     Δ                                                                           ◯                                                                      ◯                                                                      Δ                                                                            ◯                                                                      ◯                                                                      Δ                              C. Ex. 4                                                                          ◯                                                                   ◯                                                                   Δ                                                                           Δ                                                                           Δ                                                                            ◯                                                                      Δ                                                                            ◯                                                                      X    Δ                              C. Ex. 5                                                                          ◯                                                                   ◯                                                                   Δ                                                                           X   --   --   --   --   --   --                                   __________________________________________________________________________     Ex.: Example                                                                  C. Ex.: Comparative Example                                              

In Table 2, "-" means that evaluation was not carried out.

As is evident from the results of Table 2, cans made from the laminatepolyester film of the present invention are satisfactory in terms oflamination property, deep drawability, resistance to thermalembrittlement, retort resistance and corrosion resistance, andparticularly excellent in flavor retention property and impactresistance.

The total amount of oligomers contained in each of the copolyesters (I)and the amount of an extract from each of the biaxially oriented filmswith water are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Total amount of oligomer                                                                         Amount of extract from film                                contained in copolyester (I)                                                                     with ion exchange water                                    (wt %)             (mg/inch.sup.2)                                            ______________________________________                                        Ex. 1  0.40            0.35                                                   Ex. 2  0.40            0.35                                                   Ex. 3  0.40            0.35                                                   C. Ex. 1                                                                             0.40            0.35                                                   C. Ex. 2                                                                             0.40            0.35                                                   C. Ex. 3                                                                             0.50            0.45                                                   C. Ex. 4                                                                             0.40            0.35                                                   C. Ex. 5                                                                             0.40            0.35                                                   ______________________________________                                         Ex.: Example                                                                  C. Ex.: Comparative Example                                              

EXAMPLE 4 TO 9 AND COMPARATIVE EXAMPLE 6 TO 9

Copolyethylene terephthalates (intrinsic viscosity of 0.64) for thecopolyester layer (A) comprising each a copolymer component shown inTable 4 and polyester compositions shown-in Table 4 for the polyestercomposition layer (B) were, respectively, separately dried by a commonlyused method, then molten at 280° C. and co-extruded from adjacent diesto be laminated, fused together and solidified by quenching to formunstretched films. The ester exchange rate of the polyester compositionfor the layer B was set by changing the above-mentioned meltingtemperature.

Thereafter, the unstretched films were stretched to 3.2 times at 130° C.in a longitudinal direction and to 3.3 times at 120° C. in a transversedirection, and heat set at 180° C. to obtain biaxially oriented laminatefilms.

The obtained laminate films were 25 μm thick, and the copolyester layer(A) and the polyester composition layer (B) were 5 μm and 20 μm thick,respectively.

                  TABLE 4                                                         ______________________________________                                        Copolyester layer (A)                                                                                       Glass transition                                Copolymer component Melting   temperature                                     Composition    Mol %    point ° C.                                                                       ° C.                                 ______________________________________                                        Ex. 4  Isophthalic acid                                                                          9        235     74                                        Ex. 5  Isophthalic acid                                                                          9        235     74                                        Ex. 6  Isophthalic acid                                                                          9        235     74                                        Ex. 7  Isophthalic acid                                                                          9        235     74                                        Ex. 8  Isophthalic acid                                                                          9        235     74                                        Ex. 9  Isophthalic acid                                                                          9        235     74                                        C. Ex. 6                                                                             Isophthalic acid                                                                          20       208     71                                        C. Ex. 7                                                                             Isophthalic acid                                                                          2        252     76                                        C. Ex. 8                                                                             Sebacic acid                                                                              9        235     55                                        C. Ex. 9                                                                             Isophthalic acid                                                                          9        235     74                                        ______________________________________                                         Ex.: Example                                                                  C. Ex.: Comparative Example                                              

                  TABLE 4                                                         ______________________________________                                        (continued)                                                                   Polyester composition layer (B)                                               Co-PET                                                                        Copolymer                 Intrinsic  Propor-                                  component        Melting  viscosity before                                                                         tion                                     Composition  Mol %   point ° C.                                                                      blending wt %                                   ______________________________________                                        Ex. 4 Isophthalic acid                                                                         5       245    0.65     70                                   Ex. 5 Isophthalic acid                                                                         5       245    0.65     70                                   Ex. 6 Isophthalic acid                                                                         5       245    0.65     70                                   Ex. 7 Isophthalic acid                                                                         5       245    0.65     70                                   Ex. 8 Isophthalic acid                                                                         5       245    0.65     60                                   Ex. 9 Isophthalic acid                                                                         11      228    0.65     70                                   C. Ex. 6                                                                            Isophthalic acid                                                                         6       243    0.71     70                                   C. Ex. 7                                                                            Isophthalic acid                                                                         6       243    0.71     70                                   C. Ex. 8                                                                            Isophthalic acid                                                                         6       243    0.71     70                                   C. Ex. 9                                                                            Isophthalic acid                                                                         3       250    0.65     70                                   ______________________________________                                         Ex.: Example                                                                  C. Ex.: Comparative Example                                                   (Notes) PET: polyethylene terephthalate                                  

                                      TABLE 4                                     __________________________________________________________________________    (continued)                                                                   Polyester composition layer (B)  Ester                                        PBT or co-PBT                    exchange                                     Copolymer           Intrinsic    rate of                                      component      Melting                                                                            viscosity before                                                                     Proportion                                                                          copolyester                                  Composition                                                                              Mol %                                                                             point ° C.                                                                  blending                                                                             wt %  layer (B)                                    __________________________________________________________________________    Ex. 4                                                                             Adipic acid                                                                          20  195  0.90   30    7.0                                          Ex. 5                                                                             Adipic acid                                                                          20  195  1.50   30    8.0                                          Ex. 6                                                                             Adipic acid                                                                          15  204  0.90   30    6.5                                          Ex. 7                                                                             Adipic acid                                                                          25  186  0.90   30    7.5                                          Ex. 8                                                                             Adipic acid                                                                          20  195  0.90   40    7.5                                          Ex. 9                                                                             Adipic acid                                                                          20  195  0.90   30    7.5                                          C. Ex. 6                                                                          --     --  223  1.10   30    5.0                                          C. Ex. 7                                                                          --     --  223  1.10   30    5.0                                          C. Ex. 8                                                                          --     --  223  1.10   30    5.0                                          C. Ex. 9                                                                          Adipic acid                                                                          20  195  0.90   30    2.5                                          __________________________________________________________________________     Ex.: Example                                                                  C. Ex.: Comparative Example                                                   (Notes) PBT: polybutylene terephthalate                                  

Each of the films obtained in the above Examples 4 to 9 and ComparativeExamples 6 to 9 was laminated on both sides of a 0.25 mm-thick tin-freesteel plate heated at 230° C. in such a manner that the polyester layer(B) came into contact with the tin-free steel plate, cooled with waterand cut into a disk form having a diameter of 150 mm. The disk-shapedcoated plate was deep drawn in four stages by a drawing dice and punchto produce a 55 mm-diameter container (to be referred to as "can"hereinafter) having a seamless side.

The cans were observed and tested as shown in Table 5 and evaluatedbased on the following criteria. The evaluation results are shown inTable 5.

                                      TABLE 5                                     __________________________________________________________________________    Deep            Resistance to                                                                 thermal     Flavor                                                                             Taste                                        draw-     Impact                                                                              embrittle-                                                                          Retort                                                                              retention                                                                          retention                                    ability-1 resistance                                                                          ment  resistance                                                                          property-1                                                                         property-2                                   __________________________________________________________________________    Ex. 4                                                                              ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       ◯                                                                      ◯                                Ex. 5                                                                              ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       ◯                                                                      ◯                                Ex. 6                                                                              ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       ◯                                                                      ◯                                Ex. 7                                                                              ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       ◯                                                                      ◯                                Ex. 8                                                                              ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       ◯                                                                      ◯                                Ex. 9                                                                              ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       ◯                                                                      ◯                                C. Ex. 6                                                                           X    X     X     X     ◯                                                                      ◯                                C. Ex. 7                                                                           X    --    --    --    --   --                                           C. Ex. 8                                                                           ◯                                                                      ◯                                                                       ◯                                                                       ◯                                                                       X    Δ                                      C. Ex. 9                                                                           ◯                                                                      X     ◯                                                                       Δ                                                                             ◯                                                                      ◯                                __________________________________________________________________________     Ex.: Example                                                                  C. Ex.: Comparative Example                                                   (Note) --: Unable to be evaluated due to a deep drawing failure.         

EXAMPLES 10 TO 16 AND COMPARATIVE EXAMPLES 10 TO 12

Copolyethylene terephthalates (intrinsic viscosity of 0.64) for thefirst layer (A), which were prepared by copolymerizing components shownin Table 6 and a polyester composition shown in Table 6 for the secondlayer (B) were, respectively, separately dried by a commonly usedmethod, molten at 280° C. and thereafter, co-extruded from adjacent diesto be laminated, fused together and then solidified by quenching to formunstretched films.

Thereafter, the unstretched films were stretched to 3.2 times at 110° C.in a longitudinal direction and to 3.3 times at 120° C. in a transversedirection, and heat set at 180° C. to obtain biaxially oriented laminatefilms.

The obtained laminate films were 20 μm thick, and the first layer (A)and the second layer (B) were 4 μm and 16 μm thick, respectively.

                  TABLE 6                                                         ______________________________________                                               Copolyester layer (A)                                                         Copolymer Component                                                                          Melting point                                                  Composition                                                                              Mol %   ° C.                                         ______________________________________                                        Ex. 10   Isophthalic acid                                                                           12      229                                             Ex. 11   Isophthalic acid                                                                           18      214                                             Ex. 12   Isophthalic acid                                                                           6       243                                             Ex. 13   Isophthalic acid                                                                           12      229                                             Ex. 14   Isophthalic acid                                                                           12      229                                             Ex. 15   Isophthalic acid                                                                           12      229                                             Ex. 16   Isophthalic acid                                                                           12      229                                             C. Ex. 10                                                                              Isophthalic acid                                                                           12      229                                             C. Ex. 11                                                                              Isophthalic acid                                                                           12      229                                             C. Ex. 12                                                                              Isophthalic acid                                                                           12      229                                             ______________________________________                                         Ex.: Example                                                                  C. Ex.: Comparative Example                                              

                  TABLE 6                                                         ______________________________________                                        (continued)                                                                   Polyester layer (B)                                                           Co-PET                                                                        Copolymer component Melting point                                                                             Weight                                        Composition    Mol %    ° C. %                                         ______________________________________                                        Ex. 10 Isophthalic acid                                                                          6        243       70                                      Ex. 11 Isophthalic acid                                                                          6        243       70                                      Ex. 12 Isophthalic acid                                                                          6        243       70                                      Ex. 13 Isophthalic acid                                                                          18       214       70                                      Ex. 14 Isophthalic acid                                                                          6        243       70                                      Ex. 15 Adipic acid 6        243       70                                      Ex. 16 Isophthalic acid                                                                          6        243       70                                      C. Ex. 10                                                                            Isophthalic acid                                                                          4        248       70                                      C. Ex. 11                                                                            Isophthalic acid                                                                          6        243       70                                      C. Ex. 12                                                                            Isophthalic acid                                                                          6        243       55                                      ______________________________________                                         Ex.: Example                                                                  C. Ex.: Comparative Example                                              

                  TABLE 6                                                         ______________________________________                                        (continued)                                                                   Polyester layer (B)                                                           PBT or co-PBT                                                                 Copolymer component Melting point                                                                             Weight                                        Composition    Mol %    ° C. %                                         ______________________________________                                        Ex. 10 Adipic acid 20       195       30                                      Ex. 11 Adipic acid 20       195       30                                      Ex. 12 Adipic acid 20       195       30                                      Ex. 13 Adipic acid 20       195       30                                      Ex. 14 Adipic acid 10       209       30                                      Ex. 15 --          --       223       30                                      Ex. 16 Adipic acid 25       185       30                                      C. Ex. 10                                                                            Adipic acid 25       185       30                                      C. Ex. 11                                                                            --          --       223       30                                      C. Ex. 12.                                                                           Adipic acid 20       195       45                                      ______________________________________                                         Ex.: Example                                                                  C. Ex.: Comparative Example                                              

Table 7 shows the maximum peak temperature of the loss elastic modulusof each of the obtained films.

Each of the films obtained in the above Examples 10 to 16 andComparative Examples 10 to 12 was laminated on both sides of a 0.25mm-thick tin-free steel plate heated at 230° C. in such a manner thatthe polyester composition layer (B) came into contact with the tin-freesteel plate, cooled with water and cut into a disk form having adiameter of 150 mm. The disk-shaped coated plate was deep drawn in fourstages by a drawing dice and punch to produce a 55 mm-diameter container(may be referred to as "can" hereinafter) having a seamless side. Thesecans were observed, tested and evaluated.

The evaluation results are shown in Table 7.

                                      TABLE 7                                     __________________________________________________________________________    Maximum peak                                                                  temperature                                                                   of loss    Deep      Resistance  Flavor                                                                             Taste                                   elastic    draw-                                                                              Impact                                                                             to thermal                                                                           Retort                                                                             retention                                                                          retention                                                                          Overall                            modulus ° C.                                                                      ability-1                                                                          resistance                                                                         embrittlement                                                                        resistance                                                                         property-2                                                                         property                                                                           evaluation                         __________________________________________________________________________    Ex. 10                                                                            72     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      Ex. 11                                                                            70     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      Ex. 12                                                                            75     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      Ex. 13                                                                            65     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      Ex. 14                                                                            82     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      Ex. 15                                                                            65     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      Ex. 16                                                                            60     ◯                                                                      ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                      C. Ex.                                                                            80     X    --   --     --   --   --   X                                  10                                                                            C. Ex.                                                                            87     Δ                                                                            X    --     --   --   --   X                                  11                                                                            C. Ex.                                                                            47     ◯                                                                      ◯                                                                      Δ                                                                              Δ                                                                            ◯                                                                      Δ                                                                            Δ                            12                                                                            __________________________________________________________________________     Ex.: Example                                                                  C. Ex.: Comparative Example                                              

As is evident from the results of Table 7, cans made from the polyesterfilm of the present invention are excellent in deep drawability,resistance to thermal embrittlement, retort resistance and flavorretention property as well as impact resistance, particularly impactresistance at low temperatures, and does not deteriorate the taste of arefreshing drink contained therein.

We claim:
 1. A laminate polyester film to be laminated onto metal plateand molded, which comprises:(A) a first layer made fromcopoly(ethylene-terephthalate/isophthalate) having a melting point of205 to 250° C., and (B) a second layer made from a polyester (i)comprising ester units of an aromatic dicarboxylic acid component, analiphatic dicarboxylic acid component and aliphatic diol components, thearomatic dicarboxylic component being terephthalic acid, a combinationof terephthalic acid and isophthalic acid, a combination of terephthalicand phthalic acid, or a combination of terephthalic acid, isophthalicacid and phthalic acid, the aliphatic dicarboxylic acid component beingone member selected from the group consisting of adipic acid and acombination of adipic acid and sebacic acid, and the aliphatic diolcomponents being ethylene glycol, diethylene glycol, and tetramethyleneglycol, (ii) containing a titanium compound soluble in the polyester inan amount of 30 to 200 ppm in terms of elemental titanium and (iii)having one or two melting point(s) in the range of 170 to 245° C., and(C) which has a maximum peak temperature of loss elastic modulus ofhigher than 47° C. and 85° C. or lower.
 2. The laminate polyester filmof claim 1, wherein the melting point of thecopoly(ethylene-terephthalate/isophthalate) of the first layer is in therange of 210 to 245° C.
 3. The laminate polyester film of claim 1,wherein the aromatic dicarboxylic acid component of the polyesterforming the second layer is a combination of terephthalic acid andisophthalic acid, a combination of terephthalic acid and phthalic acid,or a combination of terephthalic acid, isophthalic acid and phthalicacid, and the above acid(s) other than terephthalic acid is(are) presentin a proportion of 20 mol % or less of the total of the aromaticdicarboxylic acid component.
 4. The laminate polyester film of claim 3,wherein the above acid(s) other than terephthalic acid is(are) presentin a proportion of about 1 to 12 mol % of the total of the aromaticdicarboxylic acid component.
 5. The laminate polyester film of claim 1,wherein the aliphatic dicarboxylic acid component is adipic acid.
 6. Thelaminate polyester film of claim 1, wherein the aliphatic dicarboxylicacid component is present in a proportion of 12 mol % or less of thetotal of the aliphatic dicarboxylic acid component and the aromaticdicarboxylic acid component.
 7. The laminate polyester film of claim 1,wherein ethylene glycol, diethylene glycol, and tetramethylene glycolare present in a proportion of 63 to 98.5 mol %, 1 to 2 mol % and 0.5 to36 mol %, of the aliphatic diol component, respectively.
 8. The laminatepolyester film of claim 1, wherein the second layer is prepared from amolten mixture of copoly(ethylene-terephthalate/isophthalate) having amelting point of 210 to 245° C. and copoly(tetramethyleneterephthalate/adipate) having a melting point of 170 to 223° C.
 9. Thelaminate polyester film of claim 8, wherein the molten mixture comprises99 to 60 wt % of copoly(ethylene-terephthalate/isophthalate) and 1 to 40wt % of copoly(tetramethylene terephthalate/adipate).
 10. The laminatepolyester film of claim 8, wherein the copoly(tetramethyleneterephthalate/adipate) has terminal carboxyl groups in an amount of 1 to40 eq/10⁶ g.
 11. The laminate polyester film of claim 8, wherein thecopoly(tetramethylene terephthalate/adipate) contains only 0.5 wt % atmost of oligomers.
 12. The laminate polyester film of claim 8, whereinthe copoly(ethylene-terephthalate/isophthalate) and thecopoly(tetramethylene terephthalate/adipate) have a reaction rate (EE,%), as defined by the following equation, of 3% or higher:

    EE (%)=SB/(SA+SB+SC)×100

wherein SA, SB and SC are integral values of peak A, peak B and peak Cclearly distinguishable from one another in an ¹ H-NMR chart,respectively, the peak A is based on proton H_(A) having a structurerepresented by the following formula: ##STR7## the peak B is based onproton H_(B) having a structure represented by the following formula:##STR8## and the peak C is based on proton H_(C) having a structurerepresented by the following formula. ##STR9##
 13. The laminatepolyester film of claim 1, wherein the polyester forming the secondlayer contains a titanium compound soluble in the polyester in an amountof 50 to 160 ppm in terms of elemental titanium.
 14. The laminatepolyester film of claim 1, wherein the second layer is prepared from amolten mixture of copoly(ethylene-terephthalate/isophthalate) andcopoly(tetramethylene terephthalate/adipate) and has one melting pointdifferent from the melting points of these copolyesters or has twomelting points, one of which is different from the melting points ofthese copolyesters, and the other is either one of the melting points ofthe copolyesters.
 15. The laminate polyester film of claim 1, which hasthe maximum peak temperature of loss elastic modulus of 50 to 85° C. 16.The laminate polyester film of claim 1, wherein the polyester formingthe second layer comprises copoly(ethylene-terephthalate/isophthalate)and copoly(tetramethylene terephthalate/adipate) and at least one of thepolyesters contains an antioxidant.
 17. The laminate polyester film ofclaim 16, wherein the antioxidant istetrakis-[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane.18. The laminate polyester film of claim 1 which has a thickness of 6 to75 μm.
 19. The laminate polyester film of claim 1, wherein the ratio ofthe thickness of the first layer to that of the second layer is in therange of 0.02 to 0.67.
 20. A method for using a laminate polyester film,which comprises laminating the laminate polyester film of claim 1 onto ametal plate for making a metal can by deep drawing.